Not applicable.
The present invention relates to pressure sensitive adhesives (PSA""s) based on aqueous latex emulsions and processes for the preparation of such adhesives.
Substrates (such as, for example, vinyl banners, vinyl binders, and vinyl containers) and face stocks (such as, for example, vinyl films and vinyl foams) contain plasticizers that make such products more pliable and less brittle. Often, these products are bonded with an adhesive (e.g., PSA""s) onto similar substrates, metals, wood, and other materials. Unfortunately, the plasticizers in the substrates can migrate into the adhesive causing a loss of peel strength eventually leading to a product failure (residue, loss of peel strength). In particular, plasticizers used in the manufacture of vinyl foams (e.g., dioctylphthalate or DOP and dinonylphthalate or DINP) can migrate into a bonding PSA causing the adhesive peel strength to decrease, which eventually leads to product failure. In the case of automotive foams, manufacturers"" specifications require minimum peel values over a range of environmental conditions for their PSA coated foam products. Traditional emulsion and solution PSA""s fail to meet these specifications. Typical adhesives absorb the migrating plasticizers, which results in a loss of peel strength ending in adhesion failure.
The market would like a PSA, which can meet the automotive specifications and which would be resistant to plasticizer migration, thus extending the life of a product. The market also would prefer an emulsion adhesive having plasticizer resistance due to environmental and cost constraints, because emulsions are typically non-hazardous and less expensive than other technologies (e.g., solution, or UV prepared resinous adhesives). With an adhesive that has plasticizer resistant properties, the market could produce price competitive products (e.g., foam tapes, vinyl banner graphics, vinyl labels, and comfort mats), which would have extended usefulness and durability due to their resistance to plasticizer migration.
An aqueous, pressure sensitive adhesive (PSA) resistant to plasticizer is made from an aqueous latex emulsion having an average particle size diameter of not substantially above about 100 nm and made from a mixture of ethylenically-unsaturated monomers and oligomers that include one or more of N-(iso-butoxymethyl) acrylamide, N-(n-butoxymethyl) acrylamide, and N-methylol acrylamide; and emulsified in the presence of an emulsifier consisting essentially of: 
In structure I, R1 is an alkyl, alkenyl, or aralkyl group containing between 6 and 18 carbon atoms, R2 is H or R1, R3 is H or a propenyl group; A is an alkylene group of 2 to 4 carbon atoms, n is an integer ranging from 1 to 200, X is H or SO3M, where M is an alkali metal, an ammonium ion, or an alkanolamine cation. Preferably, the ethylenically-unsaturated monomers includes 2-ethylhexyl acrylate, while 1,3-butanediol dimethacrylate can be added to increase cross-link density. Preferably, the emulsifier is: 
The preferred aqueous latex emulsion is prepared from a monomer mixture consisting essentially of at least one alkylacrylate having at least 4 carbon atoms in the alkyl chain, at least one ethylenically unsaturated carboxylic acid or its corresponding anhydride, at least one styrenic monomer, and N-(iso-butoxymethyl) acrylamide, and has a mean particle size diameter of less than or equal to about 100 nm.
The inventive PSA is designed to maintain peel performance when the PSA in use is subjected to plasticizers that migrate from the surfaces of substrates and face stocks. The inventive PSA is based on a polymer or resin, which is polymerized with special additives and a selection of monomers to give the final adhesive resistance to plasticizers, water, and other solvents. The inventive PSA further was designed using a polymerizable surfactant, which is incorporated into the polymer. Such surfactant incorporation into the cured polymer contributes to the reduction of the adverse effects (e.g., loss of peel, shear, and tack) caused by migration of and interaction with plasticizers from the substrate being adhered by such PSA.
PSA""s also need to maintain their adhesion at elevated temperatures (typically up to about 80xc2x0 C.). These elevated temperatures also increase the rate at which plasticizers migrate from the substrate or face into the PSA layer. These elevated temperatures cause prior art PSA""s to fail. The inventive PSA, however, is formulated using specialty monomers (e.g., n-isobutoxymethyl acrylamide or IBMA, and 1,3-butanediol dimethacrylate or 1,3 BDDMA) to increase the cross-linking density of the PSA polymer and, therefore, the heat aged stability of the PSA.
The inventive PSA is significantly better than PSA""s currently available, whether based on emulsion, solution, and/or UV curable technologies. The inventive PSA has excellent resistance to plasticizers. Based on the data reported herein, only small decreases (on the order of  less than 25% of initial values) in peel values are observed when the adhesive is aged at 80xc2x0 C. for 10 days. Currently available PSA""s typically lose about 80%-90% of their initial values. All other aging and cycle testing typically shows no loss in peel resulting in foam destruction for the inventive PSA, whereas other adhesives usually display a loss of around 50%-90% of initial peel values.
Referring now to the preparation of the PSA emulsion polymer, the polymerization is carried out in the presence of a reactive emulsifier or surfactant as described below. A thermal free-radical initiator system (e.g., persulfate, peroxide, or azo compound initiators) is used in an amount sufficient to promote free radical polymerization of the monomers. A redox type free-radical cure can be used advantageously to finish off the polymerization step or can be used in place of the thermal system in preparing the PSA emulsion polymer. Once the polymerization is complete it may be desirable to adjust the pH of the latex emulsion in order to enhance its stability. Other ingredients commonly used in the preparation of aqueous latex emulsions such as buffering agents, chain transfer agents, and the like may be present. General latex technology is discussed in, Kirk-Othmer, Encyclopedia of Technology, [4thEd.], vol.15, p.51-65; which is hereby incorporated by reference. In addition to the aqueous latex emulsion, the pressure sensitive adhesive may also contain additional components such as, biocides, wetting agents, defoamers, tackifiers, rheology modifiers, etc.
The reactive surfactant or emulsifier consists essentially of a compound represented by the following general structure: 
In structure I, R1 is an alkyl, alkenyl, or aralkyl group containing between 6 and 18 carbon atoms, R2 is H or R1, R3 is H or a propenyl group; A is an alkylene group of 2 to 4 carbon atoms, n is an integer ranging from 1 to 200, X is H or SO3M, where M is an alkali metal, an ammonium ion, or an alkanolamine cation. This reactive emulsifier can be made in accordance with the procedure described in U.S. Pat. No. 5,332,854, the disclosure of which is expressly incorporated herein by reference.
Preferably, the emulsifier has the following general structure: 
This preferred emulsifier is commercially available from Montello (Tulsa, OK) as Hitenol BC-10, poly(oxy-1,2-ethanediyl),xcex1-sulfo-xcfx89-[4-nonyl-2-(1 -propenyl) phenyoxy]-branched ammonium salts; yellowish brownish viscous liquid, 97.0 % actives, combined sulfuric acid content of 8.70-9.70%, pH of 6.5-8.5 (1% aqueous solution). Typically, less than 4 wt-%, and desirably less than 1 wt-%, of the reactive surfactant based on the total weight of the latex (solids basis), is used.
The reactive emulsifier can be employed with a variety of latex emulsions for formulating the novel PSA""s. The monomers used to prepare such aqueous lattices include alkyl acrylates, ethylenically unsaturated carboxylic acids and their corresponding anhydrides, and styrenic monomers.
Alkyl acrylates are alkyl esters of acrylic or methacrylic acid having at least 4 carbon atoms in the alkyl portion of the molecule. Examples include butyl acrylate, isobutyl acrylate, heptyl acrylate, octyl acrylate, iso-octyl acrylate, 2-ethylhexyl acrylate, and iso-decyl acrylate. A single alkyl acrylate or mixtures of more than one alkyl acrylate can be used. A preferred alkyl acrylate is 2-ethylhexyl acrylate. The alkyl acrylate monomers are present in the monomer mixture in an amount from about 20 wt-% to about 60 wt-% based on the total weight of the monomer mixture.
Examples of ethylenically unsaturated carboxylic acids and their corresponding anhydrides used in the present invention include acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, beta-carboxyethyl acrylate and maleic anhydride. A single ethylenically unsaturated carboxylic acid or its corresponding anhydride or mixtures thereof can be used. A preferred carboxylic acid is acrylic acid. The ethylenically unsaturated carboxylic acids or their corresponding anhydrides are present in the monomer mixture in amounts from about 0.5 wt-% to about 4 wt-% based on the total weight (solids basis) of the monomer mixture.
Examples of styrenic monomers used in the present invention include styrene, t-butyl styrene, dimethyl styrene, and vinyl toluene. A preferred monomer is styrene. The styrenic monomers are present in the monomer mixture in amounts ranging from about 5 wt-% to about 30 wt-% based on the total weight of the monomer mixture.
Additionally, it has been determined that inclusion of N-(iso-butoxymethyl) acrylamide, N-(n-butoxymethyl) acrylamide, N-methylol acrylamide in synthesizing the PSA latex, along with the reactive emulsifier results in a PSA latex that exhibits excellent resistance to plasticizers, especially at elevated temperatures. It appears that this monomer contributes cross-linking to the system to produce a polymer having a greater cohesive strength. The amount of this ingredient can range from about 0.1 wt-% to about 3 wt-% based on the total weight of the monomer mixture.
Cross-link density also can be increased by using specialty monomers, such as, for example, 1,3-butanediol dimethacrylate (1,3 BDDMA) in an amount of less than about 1 wt-%. Additionally, mercaptan chain transfer agents can be included, such as, for example, n-dodecylmercaptan, in small amounts, e.g., less than about 1 wt-%.
A thermal free-radical initiator system is used to promote polymerization of the monomers. Typical conventional initiators are used, such as, for example, persulfates, peroxides, and azo compounds. Representative such free-radical initiators include, e.g. hydrogen peroxide, sodium persulfate, potassium persulfate, and ammonium persulfate.
A redox type free-radical initiator system can be used to finish off or complete the polymerization, if desired, or it can be used instead of the thermal free-radical initiator system. The initiator may be peroxide or hydroperoxide such as t-butyl hydroperoxide. The reducing agent used in the redox system may be zinc formaldehyde sulfoxylate, sodium formaldehyde sulfoxylate, ascorbic acid, iso-ascorbic acid, sodium metabisulfite and the like. A preferred redox type system consists of t-butyl hydroperoxide and zinc formaldehyde sulfoxylate.
The aqueous latex emulsions, which form the basis of the pressure sensitive adhesives of the present invention, are prepared in a single stage synthesis with or without a seed in the reaction vessel prior to beginning the monomer feed. Reaction temperatures during the monomer feed can range from about 50xc2x0 C. to about 90xc2x0 C. A reaction vessel is charged with deionized water, an anionic surfactant and a predetermined amount of initiator. If a seed is to be used, the mixture in the reaction vessel is heated with stirring and up to 20 wt % of the pre-emulsion, more preferably up to 8 wt-% and most preferably 4 wt-% is added to the reaction vessel along with a predetermined amount of the reducing agent to form the seed. After forming the seed, the contents of the reaction vessel are heated to a desired temperature and the pre-emulsion, initiator and the reducing agent are simultaneously metered into the reaction vessel with stirring. Preferably, however, the pre-emulsion is polymerized in the vessel without use of a seed.
It has been found to be advantageous to mix the pre-emulsion and initiator. This can be accomplished by merging the pre-emulsion and initiator feed streams and passing the merged stream through a static mixer or by simply allowing the two feed streams to converge in a common feed line. Alternatively, the initiator can be added to the reactor charge resulting in a single pre-emulsion feed. On completion of the pre-emulsion feed, the contents of the reaction vessel are cooled and all ingredients are charged into the reactor. Once the reaction is complete the pH may be adjusted. The pH of the aqueous latex emulsion is preferably adjusted to a pH of about 6 to about 9 and more preferably about 6 to about 7.5. For efficiency and economy an aqueous solution of ammonium hydroxide can be used to adjust the pH. Other bases that may be used include amines, imines, alkali metal and alkaline metal hydroxides, carbonates, etc. It will be appreciated that other reaction schemes can be used in preparing the PSA latex emulsion, as those skilled in this art will appreciate.
In addition to the aqueous latex emulsion, the pressure sensitive adhesive composition advantageously contains biocides, wetting agents, defoamers, rheology modifiers, etc. Examples of suitable biocides include Kathon LX, commercially available as a 1.5% solution from Rohm and Haas and Metatin 910, commercially available from ACIMA. An example of a suitable wetting agent is Surfynol SE commercially available from Air Products, PLURONIC(copyright) type polyols commercially available from BASF Corp, and the like. Examples of defoamers include Drewplus T-1201 and Drewplus 1-191 commercially available from Ashland Specialty Chemical Company, and Rhodoline 6681, commercially available from Rhodia.
The pressure sensitive adhesives described above can be used to prepare articles such as tapes (e.g., rolls of tape), labels, signs, marking films, and the like. In a typical construction the pressure sensitive adhesive is coated or otherwise applied to a release liner such as a siliconized paper, dried, and laminated to a facestock. Alternatively, the pressure sensitive adhesive is coated directly on a facestock. Examples of facestocks include vinyl foams, cellulosics, metal foils, polycarbonates, polyethylene, polypropylene, polyethylene terephthalate, and vinyl films.
The pressure sensitive adhesives typically have a viscosity after adjusting the pH to between about 6 and about 8 of from about 100 to about 3,500 centipoises, and a non-volatiles (nv) solids content ranging broadly from about 40% to 60% and advantageously from about 45% to 55%. The pressure sensitive adhesives exhibit a shear-thinning rheology such that it allows coating even on difficult to coat films. Conventional coating techniques can be used to apply the pressure sensitive adhesives. Such techniques include dipping, slot die, air knife, brush curtain, extrusion blade, reverse roll, squeeze roll coating, and the like.
While the invention has been described with reference to preferred embodiments, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. In this application all units are in the metric system and all amounts and percentages are by weight, unless otherwise expressly indicated. Also, all citations referred herein are expressly incorporated herein by reference.